The present invention relates to a hologram recording/reproducing apparatus and a hologram recording/reproducing method, for performing hologram recording by interference of a recording beam having undergone spatial amplitude modulation with a speckle reference beam having an irregular speckle pattern.
In recent years, the holographic technology has been rapidly developed aiming at practical use of holographic memories which have been paid attention to as a strong storage candidate to compete with the next generation and the next next generation of optical disks, and a hologram recording/reproducing system for recording and reproduction of a large amount of data by utilizing the holographic technology has been proposed.
In the hologram storage based recording/reproduction system, a coherent laser beam is branched into a recording beam and a reference beam, and the recording beam is subjected to amplitude modulation according to recording data by a special light modulator (SLM). The modulated recording beam is condensed onto a hologram recording medium, when the hologram recording medium is irradiated also with the reference beam, so that the recording beam and the reference beam interfere with each other, and the resulting interference fringe is recorded on the hologram recording medium as a fine compression-rarefaction pattern.
For reproduction of the data recorded on the hologram recording medium, an illumination beam the same as the reference beam is made to be incident on the hologram recording medium at the same angle as the reference beam, whereby the data is reproduced as a diffracted beam corresponding to the interference fringe recorded on the hologram recording medium, the diffracted beam is received by an image pickup device such as CCD and CMOS, and the beam reception signal thus obtained is analyzed to thereby reproduce the data.
The storage capacity of hologram storage is determined by volume recording density, as contrasted to the storage capacity of optical disks which is determined by surface recording density. In the case of holographically recording the data, not that the recording data are recorded directly on the hologram recording medium but that an interference fringe of the recording beam and the reference beam is recorded. In the hologram storage, multiplex recording of the data is possible by utilizing a high dynamic range owing to the volume recording onto the hologram recording medium and the selectivity owing to Bragg analysis, and, by repeating the multiplex recording, a high capacity of more than several hundreds of Gbyte can be realized. Representative multiplexing systems include angle multiplexing, shift multiplexing, wavelength multiplexing, and phase modulation multiplexing.
Among the above-mentioned hologram recording multiplexing systems, the shift multiplexing is a method for performing multiplex recording by moving the recording site on the hologram recording medium in parallel little by little. In the shift multiplexing, however, a spherical wave is used as the reference beam, so that there is an anisotropy and the recording density can be raised only in one direction. Specifically, a shift selectivity of about 5 to 10 μm can be obtained in the directions in a plane including the individual optical axes of the reference beam and the recording beam, and the shift selectivity in the direction orthogonal to the plane would be about 1 mm. Therefore, in the case of a disk-like recording medium, when the shift selectivity in the circumferential direction of the recording track is set at about 5 to 10 μm to raise the recording density in this direction, the shift selectivity in the direction perpendicular to the recording track is about 1 mm, with the result that the recording track pitch is too wide to raise the recording density.
In view of the above, there is used a random phase multiplexing system for performing multiplex recording by moving the recording site on the hologram recording medium in parallel little by little while using a speckle reference beam having a speckle pattern as shown in FIG. 6. Since the wave front of the reference beam having the speckle pattern as in this case is at random, what corresponds to the shift selectivity is the autocorrelation length of the speckle pattern, which is sharp and is free of anisotropy. Therefore, where the recording medium is disk-like in shape, the autocorrelation lengths of the speckle pattern in the circumferential direction of the recording track and in the direction perpendicular to the circumferential direction can both be several micrometers, so that the recording density can be raised by narrowing the track pitch. These technologies are disclosed in the following reference: Vladimir B. Markov, James E. Millerd, and James D. Trolinger, “Multilayer holographic data multiplexing with random encoded reference beam”, Part of the Joint International Symposium on Optical Memory and Optical Data Storage 1999.Koloa, Hi.July 1999: 100–102.
However, in the case of using the random phase multiplexing system as above-mentioned, the shift selectivity is sharp, so that in the case where the optical system is vibrated, the case where the hologram recording medium is swung, the case where the hologram recording medium is replaced and the like cases, the wave fronts of the reference beam and the illumination beam would not coincide with each other, it would be impossible to obtain a reproduced image, and the stability of the hologram recording/reproducing apparatus would be spoiled.